SEC598F16
Photovoltaic Systems Engineering
Session 07
PV Systems
Solar Modules – Part 2
September 8, 2016
Session 07 content
• Solar modules
o
o
Operation
Reliability, lifetime, etc.
2
Learning Outcomes
• Introduction to module science and
technology
• Consideration of module performance,
reliability, lifetime, and decommissioning
3
PV Systems: solar module operation
But consider the case of two diodes in series
PVCDROM
PV Systems: solar module operation
Under all conditions for two solar cells connected in series (open
circuit, short circuit, or in-between)
VT = V1 + V2 and
I = I1 = I 2
PV Systems: solar module operation
For the special case of the two solar cells in series at open circuit,
V = V1 + V2 and
I1 = I 2 = 0
or
é æV ö ù
é æV ö ù
1
I sc1 - I o êexp ç ÷ -1ú = I sc2 - I o êexp ç 2 ÷ -1ú = 0
ë è Vo ø û
ë è Vo ø û
PV Systems: solar module operation
For the special case of the two solar cells in series at short circuit,
V = V1 + V2 = 0 \ V1 = -V2 and
I1 = I 2
or
é æ -V ö ù
é æV ö ù
2
I sc1 - I o êexp ç
÷ -1ú = I sc2 - I o êexp ç 2 ÷ -1ú ë è Vo ø û
ë è Vo ø û
PV Systems: solar module operation
Graphical solution of current flowing through two mismatched
diodes (short-circuit conditions)
PV Systems: solar module operation
Heat dissipated in
a shaded cell
caused the
module to crack
One shaded cell in a string reduces the current through the
good cells, causing the good cells to produce higher voltages
that can often reverse bias the bad cell.
PVCDROM
PV Systems: solar module operation
Use of a bypass diode
PVCDROM
PV Systems: solar module operation
Bypass diodes across groups of solar cells.
PVCDROM
PV Systems: solar module operation
PV Systems: solar module operation
(a) From the figure above, the sunny module short circuit current is roughly
5.8A and the shaded module short circuit current is approximately 3A. The
open circuit voltage for both modules is close to 20V. But when the two
modules are combined in series and operated in short circuit mode, then:
Vtotal = 0 = Vsunny + Vshaded
The voltages for the two modules are equal and opposite:
Vsunny = +20V Vshaded = -20V
Therefore the power dissipated in the shaded module is given by:
Pshaded = I shadedVshaded = 60W
This power is delivered to the shaded module from the sunny module, which
is generating +60W. Since they are operated in short circuit, the pair
generates net zero power!
PV Systems: solar module operation
(b)
PV Systems: solar module operation
(b) All three bypass diodes in the shaded module will be activated and
forward biased, and each diode then will have a typical silicon diode voltage
drop of 0.7 V. Therefore the three bypass diodes have a voltage drop of -2.1
V. We are still operating in short circuit conditions, so the sunny module
has a voltage drop of +2.1 V.
At a voltage of 2.1 V, the sunny module will be generating a photocurrent a
bit less than the short circuit current of 5.8 A, but let’s still call it 5.8 A. So
the sunny module will generate a power of:
Psunny = I sunnyVsunny = ( 5.8A) ( 2.1V ) = 12.2W
Therefore, in short circuit mode, the shaded module dissipates this same
amount, 12.2W. It is substantially lower than in part (a)
PV Systems: solar module operation
Bypass diodes across groups of solar cells. The voltage
across the unshaded solar cells depends on the degree of
shading of the poor cell
PVCDROM
PV Systems: solar module operation - heat
generation
For a typical commercial PV module operating at its
maximum power point, only 10 to 20% of the incident
sunlight is converted into electricity, with much of the
remainder being converted into heat:
• the reflection from the top surface of the module
• the electrical operating point of the module
• absorption of sunlight by the PV module in regions which are
not covered by solar cells
• absorption of low energy (infrared) light in the module or
solar cells
• the packing density of the solar cells
PVCDROM
PV Systems: solar module operation thermal considerations
It is desirable for modules to operate at as low a
temperature as possible
• Solar cell output is increased at lower temperatures
• Thermal cycles and stress are reduced
• Degradation rates increase with increasing temperature
S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation thermal considerations
The normal operating cell temperature (NOCT) is defined
as the temperature reached by open-circuited cells in a
module under these conditions
• Irradiance on cell surface = 800 W/m2
• Air temperature = 20o C
• Wind velocity = 1 m/s
An approximate (empirical) expression for cell temperature
(Celsius) is
é NOCT - 20 ù
Tcell = Tair ê
úû Iinc
ë
800
S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation degradation and failure modes
• Front surface soiling
o
Dust, dissolved solids, bird droppings
• Cell degradation
o
o
o
o
Decreased adherence or corrosion of contacts (increase in series
resistance
Metal migration through or around pn junction (decrease in shunt
resistance)
Deterioration in anti-reflection coefficient (reduced light absorption)
Deterioration of semiconductor material through atomic level processes
(reduced conversion efficiency)
• Module degradation
o
Discoloration of encapsulating materials
S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation degradation and failure modes, cont.
• Open-circuited cells
o
Cracked cells
•
•
•
Thermal stress
Hail, gravel, other projectiles
Damage during assembly
• Short-circuited cells
S.R.Wenham et al., Applied Photovoltaics
PV Systems: solar module operation degradation and failure modes, cont.
• Open-circuited modules
• Short-circuited modules
• Module glass breakage
• Module delamination
• Hot-spot failures
• Bypass diode failures
• Encapsulant failures
S.R.Wenham et al., Applied Photovoltaics